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EHL (Elasto Hydrodynamic Lubrication)
/ Operation
On this page: Setting-up a test | Cleaning | Maintenance
Setting-up a test
Below is a video describing how to setup a film thickness and traction test.
Cleaning
Before carrying out any measurements using the instrument it is essential that it is thoroughly cleaned. The choice of suitable solvents will depend on the materials being tested and is best left to the experience of the end user, but the following procedure has been found to work well with mineral oils:Check that the drain valve is closed.
- Check that the loading system lock is fully engaged - if the lubricant reservoir is filled with solvent with the lock disengaged solvent will drain into the interior of the instrument!.
- Remove the ball carriage by pulling it gently upwards. It may need to be rocked from side to side to break any suction between it and the top of the loading system bellows.
- Using a wash bottle, fill the lubricant reservoir to the normal operating level (the centre line of the ball drive shaft) with a suitable solvent. Toluene is often used but can cause handling problems in some laboratories because of health and safety issues. Alternatives are iso-octane or any other light hydrocarbon solvent.
- Clean the main shaft seal by squirting solvent into the seal through the cleaning holes in the main shaft flange.
- Leave the reservoir filled with solvent for 2-3 minutes then drain and refill.
- Clean around the inside of the pot and reservoir with paper wipes moistened with solvent.
- Drain the solvent then refill either with the same solvent or preferably with a more volatile solvent. DO NOT use acetone (ethanone) or any other ketone solvent as this will cause permanent damage to the Viton seals in the lubricant reservoir. Isopropanol (iso-propyl alcohol) is suitable.
- Drain and allow to dry. Drying can be accelerated by using a filtered air or nitrogen line to gently blow solvent out from the loading bellows and from the corners of the lubricant reservoir. Pay particular attention to the ball drive seal and the area around the main shaft seal as solvent can pool in these places. Blow dry the main shaft seal via the cleaning holes in the main shaft flange. Alternatively a warm air supply (<100°C) can be used to help dry the lubricant reservoir.
- Clean the test ball, disc (glass spacer layer or traction), ball carriage, glass disc centre bush, disc washer or disc cover, disc securing nut and ball drive adapter (if used) with the same solvents as used for the lubricant reservoir. The small parts are best cleaned in a beaker in an ultrasonic bath. DO NOT clean the ball carriage in an ultrasonic cleaner as this will cause damage to the carriage bearings. The disc can be cleaned with solvent moistened wipes and rinsed and dried using warm air or nitrogen as above. Clean the PTFE pot lid and plug using solvent moistened wipes.
Maintenance
The instrument has been designed to be as maintenance free as possible. There are however three areas which will require cleaning and/or replacement with time.
Traction transducer slip rings
The signals to and from the traction transducer are transmitted via a four way slip ring assembly. This is located under the black cover on top of the ball drive system. The slip rings consist of carbon/silver brushes which bear on silver rings. The brushes are designed to wear continuously to maintain good electrical contact with the rings. This means that the inside of the slip ring housing slowly gets coated with sliver/graphite dust, which if left will eventually short out the traction transducer. This will not cause damage but will lead to incorrect results when performing traction tests. It is recommended that the slip rings and their brushes and housing are cleaned once a month.
To clean the rings first ensure that the instrument is switched off. Remove the four socket screws securing the slip ring cable cover to the slip ring holder and lift the cover carefully away to expose the brush block. Remove the four button head screws holding the brush block onto the slip ring housing and lift the brush block away. Clean the brush block by blowing it clean with a low pressure air line. If the brushes have become contaminated with lubricant clean them with isopropanol and blow dry. Blow out the interior of the slip ring holder using the air line. Replace the brush block on the slip ring holder and secure with the four button head screws. Replace the slip ring cable cover and its securing screws.
Seals
There are several seals which seal various openings into the lubricant reservoir and pot. Some of these are designed to be replaced by the user of the system, some are not easily accessible and may have to be replaced by PCS personnel of suitably qualified service staff are not available to the end-user. In general if the seals are correctly lubricated as per the procedures in this manual, no problems should be encountered. The user should also note the warning in section 4 about the cleaning the instrument using acetone or other ketones. This will cause serious damage to the Viton seals in the instrument and is not covered by warranty.
The seals are:
The pot lid O ring
This is a nitrile O ring which seals the pot lid under the weight of the lid itself. It unlikely to fail.
The disc cover O ring
This is a Viton O ring which seals the disc cover to the top surface of the disc, preventing lubricant from contaminating the top surface of the disc during a test. It is unlikely to fail.
The main shaft seal
This is a Viton lip seal which seals the main shaft opening against the ingress of test lubricant. Failure will lead to lubricant escaping through the main shaft bearings into the interior of the instrument. The seal should not fail as long as it is properly lubricated before each test. Holes are provided in the main shaft flange to introduce a few drops of the lubricant under test into the seal. The user should ensure that this is done before every test. Removal of the seal involves extraction of the main shaft and will not be covered here.
The ball drive shaft seal
This is a Viton lip seal permanently mounted in a stainless steel carrier which has an external Viton O ring to seal it into the slip ring holder. Failure will lead to lubricant escaping into the interior of the slip ring housing. There is a drain hole in the bottom of the slip ring housing - the appearance of lubricant at this drain hole indicates that the ball drive shaft seal must be replaced.
Loading system base seal
This is Viton O ring located under the base of the loading system bellows. It seals the base of the lubricant reservoir under the action of the loading system lock. Failure will allow lubricant to leak down the loading system into the interior of the instrument. As long as the instructions about cleaning are followed, it should not fail.
Lubricant thermocouple seal
This is a simple nitrile sleeve which fits over the lubricant thermocouple and is compressed by the thermocouple screw to seal the space around the thermocouple.
Slip ring housing/pot seal
This is a Viton O ring located between the slip ring housing and the side of the pot. Failure would be indicated by lubricant leaking from around the join between the slip ring housing and the pot. Replacement involves removing the slip ring housing and will not be covered here.
Inlet/outlet port seals
These are Dowty type seals which fit over the threaded ends of the inlet/.outlet pipes. They should not fail.
Ball carriage bearings
The three ball carriage bearings support the ball, allowing it to rotate while transmitting the load from the loading system bellows to the ball and disc. Depending on the types of lubricant and test temperatures encountered, they will need to be replaced at intervals between monthly and yearly, whenever they show any sign of roughness or lack of free rotation. Replacement bearings are available from PCS Instruments. Removal simply involves undoing the stainless steel socket screw securing each bearing to the ball carriage. Note the position of the spacing washers on each side of the bearing. The bearing can be pulled off the securing screw and the spacer sleeve inside the bearing bore removed. Clean the new bearings with solvent (not in an ultrasonic bath) and replace, ensuring the spacing washers and internal spacer sleeve are in place.
